1. Field of the Invention
The invention relates to a piezoelectric transducer for destruction of concretions inside the body, comprising a spheroidal cap having piezoelectric ceramic elements situated on its radially inner side, hereinafter called the front side which in use faces towards the concretion to be destroyed.
2. Description of the Prior Art
German Patent Specification No. 3319871, the disclosure of which is incorporated herein by reference, discloses a transducer of the above type which comprises a mosaic of piezoceramic elements on its front side or surface with each element having a height of about 3 to about 10 mm and a lateral extension not exceeding their height. The gaps between these elements are filled with an electrically insulating material such as silicone rubber.
The excitation of a piezoelectric transducer of this kind by means of an HT pulse may have the result that an almost rectangular overpressure or underpressure pulse is generated initially depending on the direction of polarisation, the duration of which is determined by the period of propagation of the compression or expansion wave within the ceramic material. The same also occurs at the rear side or surface (i.e. the radially outer side) of the transducer. It is reflected there under phase reversal and appears subsequently with reversed phase at the front side.
An overpressure pulse is thus always followed by an underpressure pulse, and since the major proportion of the energy is also reflected at the front side under phase reversal, this action is repeated a number of times. Instead of a single pulse, what is generated is a decaying oscillation whose fundamental frequency is established by the lowest natural thickness oscillation (thickness .apprxeq.1/2 wavelength) of the piezoceramics.
It may be expected that cavitation phenomena occur in the underpressure phases of this decaying oscillation. Provided that this actually occurs on the concretion which is to be destroyed, this may lead to an accelerated destruction, and may thus have a favourable consequence. It cannot be precluded however that the cavitation threshold may already be exceeded even in the anteriorly situated tissue. Cavitation within tissue may however lead to bleeding or to tissue destruction.